1. Technical Field of the Invention
The present invention relates to a driving method of a spatial light modulator array for modulating incident light by displacing a micromirror, to the spatial light modulator array and to an image forming apparatus.
2. Description of the Related Art
In recent years, attention is paid to, as one of spatial light modulators (SLM), a digital micromirror device (DMD) in which a micromirror is formed on the basis of micro-mechanics technology and the micromirror is tilted to deflect light. The DMD is used for various purposes in the field of optical information processing, such as a projection display, a video monitor, a graphic monitor, a television and an electrophotographic print. Techniques relating to the DMD are disclosed in following patent documents JP-A-6-124341, JP-A-8-334709, and JP-A-9-238106 and the like filed by Texas Instruments Incorporated and open to the public.
In the DMD, plural micromirrors each having a size of about 16 μm×16 μm are provided at a pitch of 17 μm on a CMOS SRAM formed on a silicon substrate, and each of the micromirrors corresponds to a pixel of a screen. FIG. 27 is an exploded perspective view showing one spatial light modulator (pixel) 1 of a spatial light modulator array. A micromirror 3 is connected to a support post connection part 9 of a yoke 7 by a micromirror support post 5. The yoke 7 is held by a hinge 11.
Besides, the hinge 11 is held by a post cap 13. The post cap 13 is connected to a hinge support post connection part 19 of a common bus 17 by a hinge support post 15.
That is, the micromirror 3 is connected to the common bus 17 through the hinge 11, the post cap 13 and the hinge support post 15. A common voltage is supplied to the micromirror 3 through the common bus 17. The common bus 17 includes a landing site 21 as a stop member. The landing site 21 has insulating properties or is kept at the same potential as the micromirror 3.
Reference numeral 23a denotes one fixed electrode (first address electrode), and 23b denotes the other fixed electrode (second address electrode). The first address electrode 23a is connected to an electrode support post connection part 29 of a first address electrode pad 27a by an electrode support post 25. Besides, the second address electrode 23b is also connected to an electrode support post connection part 29 of a second address electrode pad 27b by an electrode support post 25.
A digital signal inputted from a first connection part 31a to the first address electrode pad 27a is inputted to the first address electrode 23a. A digital signal inputted from a second connection part 31b to the second address electrode pad 27b is inputted to the second address electrode 23b. The digital signals are inputted to the first address electrode 23a and the second address electrode 23b, so that the micromirror 3 is tilted, and a white display or a black display is selected. When the micromirror 3 is tilted, a part of a yoke piece 33 may come in contact with the landing site 21.
Next, the drive sequence of the spatial light modulator constructed as described above will be described.
FIG. 28 is a sectional view schematically showing the spatial light modulator shown in FIG. 27, and FIG. 29 is an explanatory view of the drive sequence of the spatial light modulator shown in FIG. 28.
In the spatial light modulator 1, the micromirror 3 is tilted to, for example, the left (the left in FIG. 28) in an initial state. At this time, as shown in FIG. 29, a given common voltage Vb is applied to the common bus 17(a). On the other hand, an address voltage Va1 applied to the first address electrode 23a is set to be smaller than an address voltage Va2 applied to the second address electrode 23b (Va1<Va2). Accordingly, a potential difference (|Vb−Va1|) at the left side of the micromirror 3 is larger than a potential difference (|Vb−Va2|) at the right side, and the micromirror is tilted to the left by an electrostatic force.
In the drive sequence to cause the micromirror 3 to transition to, for example, the right tilt state from this state, first, the voltages applied to the first address electrode 23a and the second address electrode 23b are inverted (Va1>Va2). Even if the voltages applied to the address electrodes are inverted as stated above, the micromirror 3 keeps the left tilt. This is because the right end of the micromirror 3 and the second address electrode 23b are sufficiently separate from each other, so that the electrostatic force to cause tilting is not exerted. By this operation, a so-called latch function is realized in which next writing Tw shown in FIG. 29 is efficiently enabled, while the tilting state (displaying state) is kept.
Next, the address voltages to the first address electrode 23a and the second address electrode 23b are kept as they are, and as shown in FIG. 29, only the common voltage Vb is lowered (b). Then, the electrostatic force at the left side of the micromirror 3 disappears, a slight electrostatic force is exerted at the right side, the elastic restoring force of the hinge 11 is added thereto, the left side of the micromirror 3 rises, and there occurs a state in which the holding of the left tilting is released.
Next, when the common voltage Vb is returned to the given value (c), the electrostatic force at the right side of the micromirror 3 is exerted strongly, and the micromirror 3 transitions to the right tilt state. When the micromirror 3 transitions to the right tilt state, the distance to the second address electrode 23b becomes short, so that the electrostatic force becomes relatively large, and the micromirror is now kept in a state in which the right side lands on the landing site 21. A time from the decrease of the common voltage Vb to the landing of the right side of the micromirror 3 is a switch time Tr shown in FIG. 29.
Here, the micromirror 3 receives a reaction force from the landing site 21 immediately after its right lands, so that vibration occurs. Thus, next writing (d) is performed after the switch time Tr passes and further, after a vibration damping time Ts passes. The time (Td=Tr+Ts) from the decrease of the common voltage Vb to the next writing is an intrinsic time depending on the spatial light modulator 1. Besides, in FIG. 29, Tb denotes a time from the end of the former writing to the start of the next writing. Accordingly, in the conventional driving method of the spatial light modulator array, as shown in FIG. 30, the total time (drive cycle) Tc=Tw+Tb of the writing time Tw and the time Tb from the end of the former writing to the start of the next writing is repeated, so that writing of one block (one row) BL[1] is performed, and this is performed for a specified number (M) of blocks (plural rows) BL [M], so that the display of all pixels is performed.
In the case where the foregoing spatial light modulator 1 is used to perform photosensitive material exposure at high speed, or in the case where a projector with a higher pixel number is desired to carry out a display, it is necessary to speed up the drive cycle Tc. Here, in order to speed up the drive cycle Tc, it is conceivable to shorten Tw (writing time) and to shorten Tb (time from the end of the former writing to the start of the next writing). For the shortening of Tw, from the relation of Tw=(the number of all pixels)/(writing clock frequency), reduction in the number of all pixels or speed-up of a writing clock frequency becomes effective means. However, the former is contrary to the demand for a high pixel number, and the latter depends on a clock device development technique. On the other hand, the shortening of Tb can be achieved, as shown in FIG. 31, by performing the writing during Ts (vibration damping time) (see a broken line part of displacement in FIG. 31).
However, when the writing is performed in the vibration damping time Ts (when the address voltage is inverted), there is a fear that an erroneous operation occurs according to the vibration state. FIG. 32 is an explanatory view showing a case (1) in which a normal operation occurs when the writing is performed in the vibration damping time, and a case (2) in which an erroneous operation occurs. That is, as shown in FIG. 32(1), even in the vibration damping time, in the case where for example, the right side of the micromirror 3 is in contact, even if the address voltages are inverted so that the inter-electrode voltages ΔV1=15 V and ΔV2=20 V of FIG. 32(1)A) become the inter-electrode voltages ΔV1=20 V and ΔV2=15 V of FIG. 32(1)B), the micromirror 3 keeps the right tilt. On the other hand, as shown in FIG. 32(2), in the vibration damping time, in the case where for example, the right side of the micromirror 3 is slightly separate from the landing site 21 due to vibration, when the address voltages are inverted so that the inter-electrode voltages ΔV1=15 V and ΔV2=20 V of FIG. 32(2)A become the inter-electrode voltages ΔV1=20 V and ΔV2=15 V of FIG. 32(2)B), since the right side of the micromirror 3 floats, the right electrostatic force becomes low, and the right electrostatic force becomes lower than the left electrostatic force at the time of the inversion of the address voltages, and as a result, there occurs an erroneous operation that the micromirror 3, which must be kept in the right tilt, is tilted to the left.
The invention has been made in view of the above circumstances.